There has recently been a renewed interest in the implementation of passive circuits that target communication and radar applications in the millimeter wave range. For example, is has been recognized that passive components limit the speed and frequency range of circuits at RF and higher operating frequencies. As such, at frequencies where wavelengths are shorter than 10, mm (i.e., millimeter wave or above 12, GHz for signals on a silicon chip) the signal delay over interconnections can be factored into a typical integrated circuit design. However, as frequency drops toward the lower end of the millimeter wave band and into the microwave band, passive circuit design increasingly poses challenges with respect to size. One way to overcome such issues is to incorporate slow wave structures into the device.
Slow wave structures are used in signal delay paths for phased array radar systems, analog matching elements, wireless communication systems, and millimeter waver passive devices. Basically, such structures can exhibit high capacitance and inductance, with a low resistance, per unit length. This can be advantageous to applications requiring high quality narrow band microwave band pass filters and other on chip passive elements.
In conventional slow wave structures, a single top conductor is disposed on an insulator (typically silicon dioxide) and attached to a metal ground plane. More specifically, in a conventional slow wave structure, a single path on a thick metal layer is used in a slow wave configuration where grounded or floating orthogonal metal crossing lines provide increased capacitance without affecting the inductance significantly. At the top level, due to scaling issues, the conductor signal path becomes very large, e.g., 18, microns wide and upwards of 4, microns thick. Also, in conventional applications, the conductor signal path can be vertically separated by upwards of 12, microns above the ground plane. While this transmission line is simple, it does not maximize capacitance per unit length, nor does it decrease in size.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.